1. Field of the Invention
This invention pertains to a method and apparatus for treating subterranean formations penetrated by a well. More particularly, the invention is directed to a method and apparatus by which certain portions of a perforated interval in a cased wellbore may be selectively plugged while treatment of other unplugged perforated intervals is enhanced.
2. Description of the Prior Art
It is common practice in completing oil and gas wells to set a string of pipe, known as casing, in the well and use cement around the outside of the casing to isolate the various hydrocarbon productive formations penetrated by the well. To establish fluid communication between the hydrocarbon bearing formations and the interior of the casing, the casing and cement sheath are perforated.
At various times during the life of the well, it may be desirable to increase the production rate of hydrocarbons by any of several means including acid treatment or hydraulic fracturing. If only a short, single hydrocarbon-bearing zone in the well has been perforated, the treating fluid will flow into this productive zone. As the length of the perforated zone or the number of perforated zones increases, treatment of the entire productive zone or zones becomes more difficult. For instance, the strata having the highest permeability will most likely consume the major portion of a given stimulation treatment leaving the least permeable strata virtually untreated. To overcome this problem, it has been proposed to divert the treating fluid from the high permeability zones to the low permeability zones.
Various techniques for selectively treating multiple zones have been suggested including techniques using packers, baffles and balls, bridge plugs, and ball sealers.
Packers have been used extensively for separating zones for treatment. Although these devices are effective, they are expensive to use because of the associated workover equipment required during the tubing-packer manipulations. Moreover, mechanical reliability tends to decrease as the depth of the well increases.
In using a baffle and ball to separate zones, a baffle ring, which fits between two joints of casing, has a slightly smaller inside diameter than the casing so that a large ball, or bomb, dropped in the casing will seat in the baffle. After the ball is seated in the baffle, the ball prevents further fluid flow down the hole. One disadvantage with this method is the extra expense of placing the baffle. Moreover, if two or more baffles are used the inside diameter of the bottom baffle may be so small that a standard perforating gun cannot be used to perforate below the bottom baffle.
A bridge plug, which is comprised principally of slips, a plug mandrel, and a rubber sealing element, has also been run and set in casing to isolate one zone while treating another. After retracting or acidizing the well, the plug is generally retrieved or knocked to the well bottom with a chisel bailer. One difficulty with the bridge plug method is that the plug sometimes does not withstand high differential pressures. Another problem inherent to this diverting technique is that placement and removal of the plug can be expensive.
One of the more popular and widely used diverting techniques involves the use of ball sealers. In a typical method, ball sealers are pumped into the well along with formation treating fluid. The balls are carried down the wellbore and to the perforations by the fluid flow through the perforations. The balls seat upon the perforations and are held there by the pressure differential across the perforations.
Although ball sealer diverting techniques have met with considerable usage, the balls often do not perform effectively because only a fraction of the balls injected actually seat on perforations. Ball sealers having a density greater than the treating fluid will often yield a low and unpredictable seating efficiency highly dependent on the difference in density between the ball sealers and the fluid, the flow rate of the fluid through the perforations, and the number, spacing and orientation of the perforations. The net result is that the plugging of the desired number of perforations at the proper time during the treatment is left largely to chance. It is difficult to control which perforated interval of the perforated casing will receive the balls and in many instances results in undesired stimulation in some portions of the formation.
Ball sealers having a density less than the treating fluid have been proposed to improve this seating efficiency problem. The treating fluid containing lightweight ball sealers is injected down the well at a rate such that the downward velocity of the fluid is sufficient to impart a downward drag force on the ball sealers greater in magnitude than the upward buoyancy force of the ball sealers. Once the ball sealers have reached the perforations, they all will seat and plug the perforations and cause the treating fluid to be diverted to the remaining open perforations. One problem with using lightweight ball sealers is that if the downward flow of fluid in the casing is slow, which is generally the case with matrix acidizing treatments, the drag forces exerted on the balls by the treating fluid may not overcome the upward bouyancy force of the ball sealers and thus the ball sealers may not be transported to the perforations. Another problem is controlling which interval of the formation will be treated since lightweight balls carried down the casing by the treating fluid, often plug the upper perforations before plugging the lower perforations.
In recent years, various methods and types of apparatus have been devised for introducing into the well casing ball sealers appropriate to seat upon perforations formed in the casing. The following patents illustrate various apparatus for introducing balls into the casing: U.S. Pat. Nos. 2,754,910 to Derrick et al; 3,011,548 to Holt; and 3,292,700 to Berry. While the methods and apparatus disclosed in these patents have advantages over use of plugs and packers in permitting successive zones to be treated with treating fluids, a more positive means for controlling placement of the balls is desired for ball sealers having a density less than the treating fluid density.